This invention relates to stable colloidal aqueous dispersions of partially and homogeneously crosslinked polyurethane/polyurea adhesives that are heat activatable at 70.degree. C., and have high bond strengths and good heat resistance at 70.degree. C. It relates more particularly to dispersions wherein the average size of the adhesive particles is about 0.12 micron and in which the particle size distribution is very narrow.
Chemical resistance, toughness, elasticity, and durability are desirable properties of polyurethane/polyurea resins (sometimes referred to hereinafter as PUR resins) used as adhesives and coatings for fabrics, plastics, wood, glass fibers, and metals. Good initial green strength, a low heat activation temperature, and high heat resistance are necessary properties of such resins when they are being considered for use as adhesives in the manufacture of such dissimilar products as automobiles and shoes. Prior to this invention, two-component systems (e.g., a PUR resin+an ethylene vinyl acetate polymer) have been used generally to provide the combination of high heat resistance, good contactability, good green strength, and low activation temperature required by auto manufacturers for their assembly of dashboards and door panels, and installation of carpets and headliners. External crosslinking agents usually have been required for the PUR resin adhesives of the prior art. One U.S. auto manufacturer's specifications require an aqueous dispersion of an adhesive that achieves good bond strength when activated on the substrate at 70.degree. C. without the need for an external crosslinking agent and retains that strength upon prolonged exposure to such heat.
The shoe industry requires its adhesives to have very high initial tack and green strength, quick bonding of PVC, rubber, fabric, and leather substrates, and excellent heat- and water resistance. It has been very slow to adapt to aqueous adhesives because of such requirements.
Aqueous dispersions of anionic polyurethane compositions which can be one component or two component adhesives are taught in U.S. Pat. No. 5,608,000, which is incorporated herein by reference. The polyurethane backbone of the adhesive of said patent comprises the reaction product of an isocyanate terminated prepolymer, a chain extending diamine, and a chain stopping aminoalcohol. A sulfonated polyester polyol group is incorporated into the polyurethane as an internal emulsifier. Conventional crosslinkers for isocyanate-terminated polyurethanes are taught to be less than optimal because their use requires a high level of chain extending diamine which makes it more difficult to maintain a stable dispersion. The preferred method of crosslinking, according to this patent, is by reaction at room temperature of carboxylic acid groups incorporated in the polyurethane with a crosslinker such as a polyfunctional aziridine compound.